In the future, massive bug-like aliens have invaded the Earth. But, thanks to a legendary Maori warrior, and his dazzling acrobatic skill in what looks like a left-over model from a Battlestar Galactica fan's garage sale, we defeated the bugs, and sent them packing back to their own benighted home world. But now, fifty years after the, err, future (correct tense use) does get tricky in sci-fi reviews.

I still have nightmares about writing up Looper. The bugs appear to be marshalling for a return engagement.

Back on Earth, it is decided, for reasons that make little sense, that under-13's make the best pilots and tacticians, and so a world-wide talent quest for the best, most tactically astute, and flat-out sociopathic pre-teens is underway.

This Ex-terminate Factor unearths young Asa Butterfield, playing Andrew 'Ender' Wiggin. He is a frowned-upon third child, in a society where two children is deemed the acceptable maximum. And despite his prosperous upbringing, and two utterly doting parents, Ender has grown up with a chip of pure ice on his skinny shoulders, such as makes him quite capable of kicking the living bejeebers out of boys older and apparently tougher than he.

This manifest problem child is immediately recognised as having the stuff of a nascent Napoleon or General Giap by Harrison Ford, who unexpectedly turns up in a very savvy and nicely comedic piece of casting, as the fleet commander of the earthly empire, charged with taking it to those pesky bugs.

Ford's Colonel Graff, driven by some of dodgiest theorising on child-psychology you're ever likely to hear, alternately bullies and praises Ender, hoping that the wreck he is making of the boy will be exactly the vessel of pure hatred and crystalline military brilliance the Earth needs to smash the bugs.

In short, Ender's Game is exactly the same 'nerdish-boy-saves-the-world' narrative we've been watching and reading for as long as fantasy fiction has been around, multipied by Starship Troopers, but minus that film's political satire. Obviously, nothing happens that any adult won't see coming a light-year off.

So the only really surprising thing about Ender's Game is how well the darned thing works. Asa Butterfield is heart-breakingly good as the kid with the unwanted gift, while next to him Hailee Steinfeld (True Grit) is astonishingly self-contained and assured as the best friend.

She is Hermione to Ender's Harry, but without the intrusion of teen romance, and the character works well.

Harrison Ford is an atrocious actor; surely one of the most limited since John Wayne to have ever become a movie star, but he does play one character well, and he gives a nicely curmudgeonly reading of his Han Solo/Indiana Jones persona here. And a special mention must go to Ben Kingsley, as the aging and moko'd Maori warrior Mazer Rackham: Kingsley can't decide whether to adopt a South African or Australian accent, and eventually stops trying all together. I really wouldn't want to be sitting next to Temuera Morrison if he ever sees this.